A reversible seat belt tensioner including a gearbox of the above-mentioned type is known from DE 103 42 230 A1. The gearbox includes, inter alia, a rotary element in the form of an externally-toothed gear, which upon activation of the reversible belt tensioner, is rotationally driven by a worm gear drivable by an electric motor. The rotational motion of the rotary element is transmitted via opposing coupling pawls to a belt shaft of the belt retractor upon which a seat belt is rolled in order to reduce the belt slack. The coupling pawls are each guided under tension into a guide on the rotary element. Furthermore, a friction ring is provided, which couples the two coupling pawls together and which in the event of a movement relative to the rotary element, forces the coupling pawls into a modulating movement. Due to this modulating of the coupling pawls, they are brought into engagement with a gearing of a drive gear, which drive gear is connected to the belt shaft for rotation therewith, so that the coupling pawls subsequently transfer the rotational movement of the driven gear via the drive gear to the belt shaft.
The driven gear is made of plastic, such as POM, for reasons of noise reduction, for weight reasons, and for cost reasons. With modern reversible belt tensioners, maximum tensile forces of 500-600 N can be generated in the upper end of the diagonal belt section abutting against the shoulder, which in this case are transmitted from the electric motor to the safety belt via the gearbox shaft, the externally-toothed driven gear, the coupling pawls, the drive gear, and the belt shaft. A part under particularly high load in the path of force transmission is the driven gear, which is highly loaded, especially in the region of the supporting of the coupling pawls and is thus subject to an increased probability of failure.
The object of the present invention is thus to provide a gearbox for a reversible seat belt tensioner of the above-mentioned type, wherein the probability of failure of the driven gear due to an excessive component loading of the driven gear is reduced.
To achieve this object a gearbox is proposed having the features described herein by the Figures, and the associated description.